busybox: remove bbappend

This bbappend has effectively been merged into OE-Core, although the
log buffer size is the busybox default rather than 64K - layers may
change this either by providing their own /etc/default/busybox-syslog
file (when using systemd) or modifying the
CONFIG_FEATURE_IPC_SYSLOG_BUFFER_SIZE option in busybox's build time
config.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>